Hello Tim, I do know one way that you can delete cookies and that is to get the Add-On Cookie AutoDelete by the CAD Team. Otherwise, I think it's a bug. Report it here: https://bugzilla.mozilla.org/home. Please tell me how it goes. Stay Safe!!

Hi Tim, if you have the site open currently, you can use the Storage Inspector tool -- Shift+F9 -- to view and delete site cookies.

Otherwise, hmm, are other buttons and form controls disabled, or only Manage Data?

Are you allowing Firefox to save history normally or is it set for automatic private browsing/Never remember history in the History section?

Thank you for your answers. Strangely, when I went back to the preferences page(after 10 minutes), the button had become clickable.

@Tim Awesome!